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On Mon, Mar 26, 2007 at 06:55:23PM +0200, Tzahi Fadida wrote:
> On Monday 26 March 2007 18:04, Erik Mouw wrote:
> > That's why CHS addressing has been obsoleted by LBA addressing: you
> > just tell the drive "I want sector 1234567" and it will figure out by
> > itself which sector on which track in which zone it is.
>
> That is not what i meant. Without specifying the geometry of the drive, how
> the schedulers would know accurately which sector is near which and will know
> how to order them correctly. Especially same sector position on different
> platters and sides.
The IO schedulers just assume that sectors with numbers close to each
other are indeed close on the disk. They don't know about disk geometry
and even can't know about it because it's not exported from the
drive[1]. A detailed knowledge is also not necessary because modern
drives do lots of caching and IO reordering themselves.
Even if physical geometry where exported, it would not be very useful
because the drive has two remap tables to work around bad sectors: the
factory defect list which usually contains start-length combinations of
defect physical blocks at device format time (in the factory), and the
grown defect list which usually contains LBA remappings for detected
bad blocks that happened during normal use. Incorporating that
information in the kernel would throw us back 25 years back to MFM/RLL
drives.
Erik
[1] Not completely true, you can ask a SCSI drive about it's zones
("notches" in SCSI speak), but again, the kernel doesn't.
- --
They're all fools. Don't worry. Darwin may be slow, but he'll
eventually get them. -- Matthew Lammers in alt.sysadmin.recovery
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